bootstrap: Avoid redundant compiler selection checks

In commit 6e613ff3 (bootstrap: Add infrastructure to detect threading
flags, 2017-11-28, v3.11.0-rc1~281^2~1) an extra level of nesting was
added to the selection loop, but the inner-most `break` command used to
exit the loop on success was not updated.  This caused the outer-most
loop to iterate unnecessarily and repeatedly try the same compilers
again.  In the case of compilers requiring a `-std=` flag, this may have
caused the oldest standard to be used instead of the newest.